early-init

Minimal init system to run tasks before the real init process start on embedded systems.

License

MIT

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Authors

• Alexandre Grosset
• Christophe Blaess

Summary

The aim of this small project is to easily run custom tasks during the boot of an embedded system before starting systemd, sysvinit, openrc, or any other init daemon.

The main idea is to divide the initialization tasks in two parts: the low-level tasks that need to be run at the very start of the boot, in a certain order, with a predictible behavior, and the high-level tasks that are needed to run custom code on the embedded system.

A fully predictible order of task execution is difficult to achieve with systemd for example, but systemd may be needed to run custom code with a lot of dependencies (D-bus...).

The idea is to let the kernel start early-init with the help of the init= directive of the kernel command line. Then early-init executes in order the scripts found in /etc/early-init.d/. Finally early-init gives the control to the original /sbin/init process.

Usage

Some command line options are available to configure early-init:

• The option -v (--verbose) let early-init display what it does on the standard error output.

• With the option -n (--dry-run), early-init doesn't run the scripts, only displays their names.

• The option -i <filename> (--init <filename>) allows early-init to execute an init process different than /sbin/init (for example /bin/sh for debug).

Installation

In addition to installing early-init in /sbin directory and the scripts performing the desired tasks in /etc/early-init.d/, you will need to add the init=/sbin/early-init argument on the kernel parameter line.

There is two ways to do this:

• configure the bootloader to pass this argument on the kernel command line (using bootargs variable for U-boot for example)
• configure the kernel himself to add the parameter on the command line.

The second way is much easier, as it only needs to configure three kernel options:

• CONFIG_CMDLINE must be filled with the string "init=/sbin/early-init"
• CONFIG_CMDLINE_EXTEND must be enabled (y)
• CONFIG_CMDLINE_FROM_BOOTLOADER has to be disabled.

We can achieve this with a simple kernel configuration fragment (present in the /cfg subdirectory of the project):

early-init-fragment.cfg: 

CONFIG_CMDLINE="init=/sbin/early-init"
CONFIG_CMDLINE_EXTEND=y
# CONFIG_CMDLINE_FROM_BOOTLOADER is not set

But this works only on some architectures (arm, riscv, powerpc...) but not on arm64 for example. So most of the time you will need to add the init= parameter from the bootloader configuration.

One possible way for U-boot is to add a boot.cmd file on the boot partition, containing at least this line:

setenv bootargs ${boootargs} init=/sbin/early-init

Examples of use

There are some scripts provided as examples in the early-init.d subdirectory of the project:

• 010-mount-data-partition.sh is a script to mount in read-write mode the data partition, repariring the filesystem in case of mount error, and reformating the partition if the repair is not possible.
• 020-mount-overlayfs-on-etc.sh allows to mount a (read/write) overlays on the (read-only) /etc directory. The configuration modifications will be stored on the /data partition.
• 030-system-time-from-rtc.sh is a one-liner script to set the system date and time from the RTC.

Here are some other ideas of tasks that may need to be run as soon as possible at boot time and in a given order:

• loading needed kernel modules with modprobe,
• reading a configuration file in /etc/ then setting network interfaces up,
• connecting to a remote NTP server and setting exact system time,
• ...

Yocto Project recipe

A recipe for Yocto Project is present in the cfg/ subdirectory of the project.

This recipe is mainly intended as an example and you probably will need to customize the list of the task scripts to be installed.